Means and method of producing flat surfaces



March 26, 1940. HQLMAN 2,194,551

MEANS AND METHOD OF PRODUCING FLAT SURFACES Filed June 11, 1937 BY MATTO RN EY Patented Mar. 26, 1940 A UNITED STATES MEANS AND METHOD OFPRODUCING FLAT SURFACES Herbert Edward Holman, Hayes, England, assignorto Electric & Musical Industries, Limited, v Hayes, Middlesex,

company England, a British Application June 11, 1937, Serial N...147,625

' In Great Britain June 25, 1936 4 Claims.

This invention relates to methods of producing flat surfaces and whilecapable of general application it has particular reference to theproduction of fiat electrodes or electrode base sup-- 5 porting, orseparating, members such as are used in cathode ray oscillograph tubes,thermionic valves, light sensitive cells and similar devices.

Cathode ray tubes used for television transmission purposes may containin addition to a cathode, accelerating and focusing electrodes, a fiatelectrode in the form of a mesh screen which acts as a stabilizing or ascreening electrode, or again a flat activated mica sheet may act as asignal producing electrode as in the case of the type of tube known asan iconoscope. Again,

such a tube may have a flat signal plate for collecting electronsemitted from an activated mica or other sheet. Such electrodes arefrequently mounted extremely close to each other and it is desirabletherefore that they should be as flat as it is practically possible tomake them, and accordingly it is the chief object of the presentinvention to provide a method of producing a flat electrode or electrodebase.

According to the present invention a method of flattening a piece ofmaterial and maintaining said material substantially flat, comprisessubjecting said piece of material to a stretching treatment, securing itto a supporting framework while in the stretched condition andsubsequently severing the portion mounted upon the supporting frameworkfrom the body of the material.

The stretching treatment may consist in ap- "plying said material to anadjustable stretching device, securing it to a supporting frameworkwhile in the stretched condition and subsequently severing the portionmounted upon the supporting framework from the body of the ma- 40terial.

Again the stretching treatment may consist in applying said piece ofmaterial to a heated surface and while in the heated condition applyinga temporary supporting framework to said piece of material with acementing medium interposed-between said material and framework,allowing the assembly to cool and thereafter mounting said piece ofmaterial upon a final supporting framework and severing the portion 5mounted on this final supporting framework from the portion cemented tosaid temporary framework.

In a particular application of the invention, a flat electrode orelectrode supporting or base member for use in a device of the typereferred to is produced by applying it to the adjustable stretchingdevice, securing it to a supporting framework while in the stretchedcondition and subsequently severing the portion mounted upon thesupporting framework from the portion ap- 5 plied to the adjustablestretching device. The adjustable stretching device may compriserelatively movable elements to the surfaces of which an adhesivematerial is applied which serves to maintain the electrode or othermember in posi- 10 tion during the stretching operation while permittingrelative movement between parts of the electrode or other member and theengaged surface of the adjustable stretching device, and the supportingframework may be secured to the IE electrode or other member by anelectroplating operation. A metal mesh electrode or a sheet of mica maybe mounted upon a metal support ing framework having a lower coefficientof ex-- 'pansion than the metal mesh and in a particum lar applicationof the invention, the metal framework is of the low expansion steelknown under the registered trade-mark'Invar.

In order that the invention may be more clearly understood and readilycarried into efiect, the 35 production of a fiat mesh electrode and of aflat mica base for an activated screen will now be described by way ofexample with reference to the drawing in which:

Figure 1 is a plan of stretching device em- 3 ployed in carrying out theinvention,

' Figure 2 is an enlarged view of the movable elements of the stretchingdevice and shows a piece of mesh in position,

Figure 3 is an end elevation of Figure 2, w

Figure 4 is a fragmentary view ofa piece of mica sheet prepared prior toapplication to the stretching device.

Figure 5 is a side view of a modified f arm of stretching apparatus and,m

Figure 6 is a planned view of Figure 5.

Referring to Figure l of the drawing, the stretching device consists ofa fairly. thick base member I of a substance which is unaffected byacids or alkalies and may be of the material Q5 known under the registertrade-mark Paxolin. Four bars or elements 2 of this material arearranged to be relatively movable over the surface of the base i bymeans of nuts 3 and screw rods 4 the ends of which are secured tothebars 5 2. Adjustment of the nuts 3 causes the bars or elements to bedrawn towards' cr away from. each other. Set screws 5 serve to clamp thebars 2 in any desired position. Further screws 6 shown in Fig. 3 serveto raise the level of supw porting framework 1 which is placed withinthe that the margin of the mesh is held by the var nish. The device isthen left in order to allow the varnish partially to harden. When thevarnish is sufiiciently hard the bars or elements 2 are moved slowlyaway from each other to tension the mesh in bi-lateral directions byadjustment of the nuts 3, the mesh being stretched to an extentdependent upon the viscosity or molecular friction of the semi-hardvarnish. The uniform viscosity of the varnish allows it to yield atpoints where further tension would rupure the mesh while permittingstretching of the mesh where further extension is possible. Further, anycross stresses which may be introduced due to the pulling of the mesh inonly two directions, i. e. bi-laterally will be eliminated by movementof the mesh along its adhering'edges, the degree of tension being thusequalized from point to point. It is desirable to perform the stretchingoperating in stages allowing a period mesh has been obtained, theframework 1 is j raised into contact with the mesh by operating thescrews 6; 4

The complete device is now placed in an electro-plating bath and metalis plated on the framework 1 through the spaces in the mesh 8, theframework 1 being connected to the negative terminal of the platingapparatus. After the plating process has continued for a sufficient timethe spaces in the mesh become filled with deposited metal which formsrivets firmly securing the mesh to the framework 1. It is important thatthe mesh within the framework should not become filled with depositedmetal and accordingly this area of the mesh is treated with a varnishwhich prevents the plating solution from affecting the mesh where thevarnish is applied to it.

Finally the -mesh is cut around the outside edge of the framework whichis then removed from the stretching device and the surplus meshremaining on the elements or bars 2 is removed.

The framework 1 may be of various metals but in all cases where the meshis likely to be heated when mounted in a discharge tube, it is preferredto use the low expansion steel known as Invar. While at first sight itmight appear desirable that the metal supporting framework should havethe same co-efiicient of expansion as the mesh, this would only be thecase if the mounted mesh could be heated and cooled slowly. Assumingthat the framework and mesh have the same ooefficient of expansion,because the mesh cools much more quickly than the supporting frameworkthere would be a resultant straining of the mesh.

the mesh will become slack on heating because it A thin will expand morerapidlythan the framework, but on cooling, the straining will cease, theframe work retaining substantially its original size. The elasticity ofthe mesh will usually be quite sufficient to absorb the slightdifference in the size of the framework.

Referring to Fig. 4 of the drawing a fragment of a mica plate 9 intendedto form a base for a mosaic electrode is shown. The method of mountingsuch a mica plate upon a supporting framework is similar to that alreadydescribed,

but as the mica has a continuous surface, three rows of holes in, II andI2 are provided. The first row of holes in is provided to facilitaterapid hardening of the gold size which would not otherwise be exposed tothe air. The holes l0 cause the formation of plugs of gold size in themica plate and oxidation of the size is expedited in consequence of theincreased exposed area. The row of holes ll permit deposition of metalupon the supporting framework and the formation of rivets aspreviouslyvdescribed. The row of holes I2 is provided to facilitatefinal cutting of the surplus mica away from that mounted upon theframework.

A modified method of mounting a mica sheet is illustrated in Figures 5and 6 of the drawing. Referring to these figures, a rectangular metalframework 13 of square section and about 1 centimeter larger than theframework ,to whichthe mica sheet is finally secured is used to providea temporary supporting framework for the mica between a stretchingoperation and the final riveting by'electro-plating. A sheet of mica Mperforated as described with reference to Figure 4 is placed upon anaccurately planned steel plate l5 which has beencarefully cleaned andthe plate with the mica is heated to a temper ature of about 150 degs.C. The framework i3 is ground flat on one side and has a coatingof a waxcomposition, the melting point of which is approximately 150 degs. C.applied .to the ground side which is then placed upon the heated micasheet on the steel plate and pressed firmly into contact with themicasheet. bly is now allowed to cool, and in order tokeep the micasheet fiat a block of metal 16 is placed on the mica within theframework 13 during the cooling process. i

As the mica sheet is in an extended condition when the metal frameworkis placed in position,

The assemit is clear that when the mica cools it will be in a state oftension, the framework, as previously mentioned, being of the metalknown under the registered trade-mark Invar, and having a lowerco-efficient of expansion than the mica. degree of tension is controlledby the solidifying The temperature of the wax used to cement theframework to the mica andthe temperature to which the mica is raisedprior to the framework being placed upon it.

The stretched mica sheet attached to the framework is now placed on thefinal supporting framework and the assembly is. placed in anelectro-plating bath in which the mica becomes riveted to the finalsupporting framework as described above.

While gold size has been referred to as a suitable varnish, it willbeunderstood that other materials may be employed. Thus cellulose acetate,

the preparation sold under the trade name Durofix, bitumen compounds,marine glue, various aquarium cements, and so forth, may all be used,but in these cases the process of tensioning needs to be carefully andprogressively performed during the whole period of the drying of thecement in order that the edge of the mesh may move endwise as isnecessary in order to ensure fiatness. Progressive tensioning is, ofcourse, necessary with any cementing medium employed, but owing to thefact that gold size is somewhat elastic when dry, the need for'exactnessis not so great as when other and less elastic cements are used. It isdesirable that the cementing material should be one in which thehardening is 7 brought about by the oxidation of the vehicle in whichthe gums or resins are dissolved. It is easier to control the degree ofhardening produced in a given time in the case of varnishes of the typereferred to than in the case of varnishes hardened by heating orevaporation which will not allow satisfactory'manipulation.

The use of water soluble cements or glues is not possible when theprocess of securing the mesh to the mounting ring by electro-depositlonis required, but if other means of securing are employed, such assoldering by fusible metals,

such adhesives would be suitable providing the tensioning is carefullyattended.

Again, any material may be used which permits relative movement betweenthe material to be stretched and the surface of the stretching frameworkwhile maintaining a substantial gripping action. Thus, as a furtherexample, aviscous non-drying material such as rubber grease underpressure from a clamping device may be tions may be made within thepurview of this invention and that such modifications shall beconsidered to be within the spirit and scope of the appended claims.

I claim as my invention:

1. The method of flattening a piece of material and securingthe same toa frame which comprises the steps of elastically attaching the materialto a stretching device, bilaterally tensioning the material by movementof the stretching device, permanently securing the frame to the materialwhile the latter is under tension, and subsequently severing the portionmounted upon the frame from the portion attached to the stretchingdevice.

2. The method of flattening a piece of material and securing the same toa frame for subsequent use in an electronic discharge device whichcomprises the steps of elastically attaching the material to astretching device, uniformly tensioning the material in bilateraldirection by movement of the stretching device, permanently securing aframe having a low coefficient of expansion to the material while thelatter is under tension, and subsequently severing that portion of thematerial which extends beyond the periphery of the frame from the partsecured to the frame.

3. A device for flattening a piece of material comprising a base member,a plurality of slidable elements positioned on said base, means forelastically securing the material to said slidable elements, and meansfor sliding said elements whereby the material may be uniformlytensioned due to such sliding and the elasticity of the securing means.

4. A device for flattening a piece of material prior to its beingsecured to a frame comprising a base member, a plurality of angular-1ypositioned adjustable elements positioned on said base, means forelastically securing the material to said adjustable elements, a framemember, means for moving the adjustable elements whereby the materialmay be uniformly tensioned due to such movement and the elasticity ofthe securing means, and means for maintaining said frame member incontact with the tensioned material whereby the material maybepermanently secured thereto.

HERBERT EDWARD HOLMAN.

